This invention relates to a repeater system for enabling mobile or portable radio stations to communicate with each other and, in particular, to a communication system that increases usage of existing channels in a band without increasing conflicting requests or "collisions."
The frequency spectrum is like land, minerals, or fossil fuel, in that it is a finite resource. Meetings are held internationally to allocate portions of the spectrum among the many claimants. Various segments or bands of the frequency spectrum are dedicated to mobile communications, typically using a repeater. Other bands are allocated to federal, state, or local governments, amateur radio, and utilities, for example. Recently, the government of the United States discovered that auctioning licenses to operate in portions of the frequency spectrum was a spectacular source of revenue. Providers of cellular telephone and other communication services anxiously purchased as many site licenses and channels as possible, viewing the licenses as an investment. The view is reasonable because, once taken, there are no more channels available.
A repeater is a receiver-transmitter combination for receiving a signal at one frequency and simultaneously re-transmitting the same signal on a second frequency. Depending upon application, the transmitted frequency may be relatively close to the received frequency, e.g. 600 khz., or greatly displaced from the received signal. Depending upon application, frequency, and government regulation, the transmitter in a repeater may be relatively powerful, hundreds of watts, or may be rated at just a few watts.
Commercial two-way radio communication has evolved into two different techniques for mobile operation, cellular and specialized mobile radio (SMR) or dispatch service. Cellular systems use several repeaters dispersed in a geographic area and operating at low power to keep propagation relatively short, e.g. within a radius of less than ten miles. The local area covered by each repeater overlaps the local areas covered by neighboring repeaters, forming overlapping "cells" of coverage. A subscriber traveling from one cell to another cell is automatically switched from one repeater to another by a computer coupled to the repeaters by microwave link, optical fiber, or wire.
Because propagation is short, the frequencies used by one repeater can be used by a non-neighboring repeater without interference, thereby serving more subscribers in a given geographic area. Re-using assigned frequencies in a geographic area provides much more efficient use of the limited resource.
SMR uses a powerful repeater, usually located at the highest available elevation in a geographic area. The repeater is coupled to an omni-directional antenna to cover the entire geographic area, enabling dispatchers to communicate with a fleet of vehicles in the geographic area and enabling the vehicles to communicate with each other. U.S. application Ser. No. 498,510, filed Jul. 5, 1995, and owned by the owner of this invention, discloses a system for increasing use of an SMR repeater by defining a plurality of cells radially extending from the antenna.
Despite these techniques in the fields of cellular radio and SMR, the number of channels available will become less than the number needed for all potential users. Providing service even for current users requires a large number of repeaters, which are very expensive to install and to maintain. Part of the problem is that repeaters available today operate on a small number of frequencies and are designed to handle the maximum anticipated traffic. As such, many repeaters are often idle for long periods when traffic is light. The problem is compounded if a repeater is held in reserve to cover failures due to lightning or some other cause. This further increases the number of repeaters required and, by definition, the reserve repeater is idle.
In view of the foregoing, it is therefore an object of the invention to increase the utilization of existing mobile communication equipment.
A further object of the invention is to provide a communication system that makes more efficient use of allocated channels and transmission sites.
Another object of the invention is to provide a communication system that can serve more than one licensee.
A further object of the invention is to provide a system that enables small service providers to increase the number of subscribers that can be serviced.
Another object of the invention is to provide a repeater back-up function without requiring an additional repeater.
A further object of the invention is to increase usage of relatively idle channels in a communication service area.